Wavelength division multiplexing (WDM) is a technique for increasing the capacity of existing fiber optic networks by transmitting a plurality of channels over a single waveguide medium. WDM systems typically include a plurality of transmitters for transmitting modulated information signals on a designated one of a plurality of optical channels or wavelengths. The channels are combined by a multiplexer at a first terminal and transmitted to a demultiplexer at a receiving terminal where the channels are separated. One or more amplifiers positioned along a transmission fiber may be used to amplify the transmitted signal. The separated channels are then supplied to receiving circuitry which converts the optical signals into electrical signals for processing.
The transmitters used in WDM systems typically include semiconductor lasers each transmitting on a designated one of a plurality of wavelengths typically within the 1550 nm range. The output signal of each semiconductor laser is controlled by an associated drive current such that the transmitter output is locked to a particular channel wavelength and modulated with communication information either directly or externally. Since the transmitted wavelengths are relatively close to each other, the laser transmitters must be precisely controlled in order to insure integrity of the communication information. Channel spacings in a WDM system are typically within the 100 GHz range. With the employment of more advanced wavelength separation devices, 50 GHz and even 25 GHz spacings may also be employed. Because these channel spacings are so close together, if the laser transmitter should lose lock during operation, the output signal of one channel may interfere with an adjacent channel, thereby corrupting the transmitted communication signal.
When a laser transmitter used in a WDM system is first turned on, it experiences a ramp-up period where the drive current must first increase to a level where the semiconductor laser provides light at the desired wavelength and at peak power. During this ramp-up period, the laser transmitter is not yet operating at the desired wavelength, however light still propagates down the transmission fiber. In addition, during laser transmitter operation, the laser output may drift off-channel allowing unwanted light to propagate down the fiber. In either case, this unwanted light transmission may impact adjacent channel transmission and/or may compromise the integrity of the transmitted information signals.
Thus, there is a need to provide an optical device which locks the wavelength output of a laser transmitter in an optical transmission system as well as being configured as a self filtering device for preventing unwanted light signals to propagate down a transmission fiber.